Thin film thermocouples have several advantages which have led to their use in numerous interesting applications. Their small size and low profile (1 .mu.m) permit temperature measurements in such difficult environments as gas turbine engines and in diesel engine cylinders. The small size of the sensor not only permits excellent spatial resolution, but also permits extremely fast response (1 .mu.s) in measuring the surface temperature of material in contact with hot and corrosive gases. The unique properties of thin film thermocouples combined with their low cost and suitability for automated production has led to wider use and expanding applications.
Many applications, particularly in the aerospace industry, involve high temperature environments where stability and reliability under harsh conditions are principal design considerations. A review of candidate materials for surface temperature measurements up to 1650.degree. C. (3002.degree. F.), which includes a detailed analysis of potential materials and techniques for the measurements together with discussions of some of the compatibility problems, has been published by Bennethum et al., in "Sensors for Ceramic Components in Advanced Propulsion Systems", NASA Contractor Report 180900, NASA Lewis Research Center, Cleveland (1988).
The demanding requirements of high temperature, corrosive environments have led to the use of platinum-rhodium alloy thermocouples for their outstanding resistance to oxidation at high temperatures. Recent studies of thin-film platinum/platinum rhodium thermocouples have indicated serious oxidation problems related to rhodium oxidation in the 700.degree.-900.degree. C. range and substrate reactions at temperatures above 1250.degree. C. See Kreider, J. Vac. Sci. Tech., A11 (4) July/August 1993 and Hollanda, Temperature: Its Measurement and Control in Science and Industry, Vol. 1, p. 649, Am. Inst. of Physics, N.Y. (1992). The reactivity of the thermoelements is a more profound problem with thin films than with wires because of the very short (.ltoreq.1 .mu.m) diffusion distances. What might be a small surface reaction layer on a 0.5 mm (500 .mu.m) wire can consume a 1 .mu.m thick film. In alloy thermocouples the alloying element, for example-Rh, had different reactivities with environmental factors, such as oxygen, sulfur, or silicon, than the platinum and a change in the solute levels leads to changes in the Seebeck coefficient.
Refractory metal silicides have been used commercially in high temperature air furnaces as heating elements and in fixtures. MoSi.sub.2 is presently used in the highest temperature commercial electrical heater elements, which are commonly used to 1600.degree. C. in air. Metal silicides have also been considered as possible electrical conductors in integrated circuits, and their properties, thermodynamics, formation and oxidation have been studied, and silicide characteristics and mechanisms are described in the literature in connection with silicide conductors on silicon. See S. P. Murarka, Silicides for VLSI Applications, Academic Press, N.Y. (1983).
Flais et al., U.S. Pat. No. 3,767,469, has even proposed the use of MoSi.sub.2 as a thermoelement of a thick-film thermocouple, but has to use a sealed reference electrode in order to eliminate the effects of oxygen migration.
Despite the efforts of the prior art, there has remained a need for improved thin film thermocouples which can be used particularly in high temperature environments and/or under oxidative and/or corrosive conditions.